feat: tuple syntax cutover — Tuple(...) type + .(...) value

Replace the bare-paren tuple grammar with explicit, position-unambiguous
forms, mirroring how structs work:

  type     `(A, B)`        -> `Tuple(A, B)`          (named keeps `:`)
  value    `(a, b)`        -> `.(a, b)`              (named uses `=`)
  typed    (new)           -> `Tuple(A, B).(a, b)`   (like `Point.{...}`)
  failable `-> (T, !)`     -> `-> T !`
           `-> (T1, T2, !)`-> `-> Tuple(T1, T2) !`   (channel outside Tuple)

Bare `(...)` is now grouping only, everywhere; a comma in bare parens is a
hard error with a migration hint. Grouping, function types `(A, B) -> R`,
param lists, lambdas, and match bindings are unaffected.

`Tuple(...)` is strictly a TYPE in every position (including `size_of` /
`type_info` args); a tuple VALUE comes only from `.(...)` (anonymous) or
`Tuple(...).(...)` (explicitly typed). A bare `Tuple(1, 2)` is a tuple
type with non-type elements -> rejected.

The ~110 tuple-bearing corpus files were migrated with a one-shot
AST-aware migrator (the `sx migrate` tool from the prior commit, removed
here). New examples: 0130 (new syntax), 0131 (typed construction), 1060
(named-tuple failable return). 1116 golden updated for the new hint text.

examples/diagnostics/1101-diagnostics-err-tuple-oob.sx

@@ -2,6 +2,6 @@
 // `error: field 'N' not found on type '(i64, i64)'` diagnostic and exit 1.
 
 main :: () -> i32 {
-    t := (10, 20);
+    t := .(10, 20);
     return xx t.42;
 }

examples/diagnostics/1116-diagnostics-tuple-type-nontype-element-rejected.sx

@@ -8,6 +8,6 @@
 #import "modules/std.sx";
 
 main :: () -> i32 {
-    print("bad tuple type size = {}\n", size_of((i32, 1)));
+    print("bad tuple type size = {}\n", size_of(Tuple(i32, 1)));
     0
 }

examples/diagnostics/1121-diagnostics-reserved-name-control-flow.sx

@@ -11,7 +11,7 @@
 // offending name; exit 1 — NOT an LLVM verifier abort.
 #import "modules/std.sx";
 
-pair :: () -> (i64, i64) { (1, 2) }
+pair :: () -> Tuple(i64, i64) { .(1, 2) }
 maybe :: () -> ?i64 { return null; }
 
 main :: () -> i32 {

examples/diagnostics/1124-diagnostics-imported-reserved-destructure/mod.sx

@@ -1,6 +1,6 @@
 #import "modules/std.sx";
 
-pair :: () -> (i64, i64) { (1, 2) }
+pair :: () -> Tuple(i64, i64) { .(1, 2) }
 
 run :: () -> i32 {
     i2, rest := pair();   // destructure name in an IMPORTED module

examples/diagnostics/1157-diagnostics-catch-binding-needs-parens.sx

@@ -6,7 +6,7 @@
 
 E :: error { Bad };
 
-f :: () -> (i64, !E) { raise error.Bad; }
+f :: () -> i64 !E { raise error.Bad; }
 
 main :: () {
     v := f() catch e { 0 };

examples/diagnostics/1202-diagnostics-null-coalesce-tuple-default.sx

@@ -8,7 +8,7 @@
 #import "modules/std.sx";
 
 main :: () {
-    o : ?(i32,) = null;
+    o : ?Tuple(i32) = null;
     x := o ?? 5; // default 'i64' vs payload '(i32,)' -> diagnostic
     print("{}\n", x.0);
 }

examples/diagnostics/expected/1116-diagnostics-tuple-type-nontype-element-rejected.stderr

@@ -1,5 +1,5 @@
-error: tuple type element is not a type (found `int_literal`); a tuple used as a type must list only types, e.g. `(i32, i32)`
-  --> examples/diagnostics/1116-diagnostics-tuple-type-nontype-element-rejected.sx:11:55
+error: tuple type element is not a type (found `int_literal`); a tuple used as a type must list only types, e.g. `Tuple(i32, i32)`
+  --> examples/diagnostics/1116-diagnostics-tuple-type-nontype-element-rejected.sx:11:60
    |
-11 |     print("bad tuple type size = {}\n", size_of((i32, 1)));
-   |                                                       ^
+11 |     print("bad tuple type size = {}\n", size_of(Tuple(i32, 1)));
+   |                                                            ^